1. Field of the Invention:
The present invention relates to improvements of spatial light modulator capable of using a high intensity reading light.
2. Description of the Related Art:
Spatial light modulators (SLM) can transform an incoherent light into a coherent light, or vice versa. Application of spatial light modulators to parallel processing of data and to direct arithmetic processing of image has been discussed. If the intensity of light can be amplified, spatial light modulators can be applied in display systems such as video projectors.
Various spatial light modulators of this kind are known as discussed in the APPLIED PHYSICS LETTERS, Vol.22, No. 3, Feb. 1, 1973, pp. 90-92, Preprint for the 50th Meeting (Autumn 1989) of the Japan Society of Applied Physics, 28P-ZD-5-7.
One of such known spatial light modulators is shown as SLM in FIG. 1, in which a first transparent electrode 18, a photoconductive layer 16, a light-blocking layer 14 which is electrically nonconductive, and a dielectric mirror 12 are laminated by deposition on a glass substrate 20 in this order, further, a photomodulation layer 10 using a liquid crystal as a modulation material, is laminated over the dielectric mirror 12, a beam of writing light F1 impinges one side of the photomodulation layer 10 through the above laminated structure from the side of the glass substrate 20.
On the other hand, a second transparent electrode 22 and a glass substrate 24 are laminated on another side of the photomodulation layer 10 on which a reading light F2 impinges. An appropriate driving power supply 26 is connected between the first and second transparent electrodes 18 and 22.
The operation of the spatial light modulator SLM of above structure is now described briefly. The beam of writing light F1 carrying information to be written passes through the glass substrate 20 and the transparent electrode 18 and enters the photoconductive layer 16. As a result, electron-hole pairs are generated in the photoconductive layer 16 corresponding to the intensity distribution of the writing light F1 thereon. Then, these pairs are separated to form an image of electric charge corresponding to the distribution of the intensity of the beam of writing light F1.
On the other hand, the reading light F2 is projected to the photomodulation layer 10 through the glass substrate 24 and the second transparent electrode 22, where the photomodulation layer 10 is subject to an electric field generated by the charge image formed in the photoconductive layer 16 as described above. When the reading light F2 passes the photomodulation layer 10 and reflected back by the dielectric mirror 12 and passes again the photomodulation layer 10 as a reflected light F3, an optical modulation is performed on the beam of reading light F2 modulated in accordance with a two dimensional field strength distribution of the charge image i.e. according to the information carried by the writing light F1.
A function of the light-blocking layer 14 is to prevent the reading light F2 from reaching to the photoconductive layer 16, otherwise the image of electric charge would be disturbed, leading to a reduction in the contrast of read out image.
When such spatial light modulator is used for a video projector, the intensity of the reading light F2 would be much higher than that in the other applications, the higher the intensity of the reading light the more critical thus significant the function of the light-blocking layer 14. For better resolution, the light-blocking layer 14 should be highly resistive electrically and have a high absorption coefficient, which restrict material selections. One of known light blocking films is a CdTe film, but an adequate quality CdTe film is not easily available and is poisonous, a potential problem. There exists a different type of light-blocking film of multiple layers utilizing an optical interference such as disclosed in the Preprint for the 51st Meeting (1990) of the Japan Society of Applied Physics, p. 751 26a-II-3. This type is very effective only in the perpendicularly incident light, but is much less efficient in the obliquely incident light such as from the photomodulation layer 10 of scattering type such as polymer-dispersed liquid crystal. Further, this multi-layer light-blocking film utilizes the interference of light and is wavelength dependent, thus is ineffective in the broad band light when used with a TN-type liquid crystal device.
In the spatial light modulator SLM, the photomodulation layer 10 is driven according to the distribution of the field strength due to the charge image which corresponds to the conductivity variation in the photoconductive layer 16, under such circumstances, if the electrical resistance (a degree of insulation) of the light-blocking layer 14 and/or a reflective layer to reflect the reading light F2 were not high enough, the distribution of the generated electric field would be two dimensionally diluted or deteriorated in a two dimensional directions along the plane (hereinafter called "intraplane") of the photomodulation layer 10 causing a loss of image resolution in the photomodulation layer 10. This is the reason why the mirror 12 is made of dielectric material and the light-blocking film has to be highly resistive.
On those films, the intraplane resistance is the most significant factor for the image resolution, therefore, a simply high insulation in the thickness direction (in the direction perpendicular to the surface of such film) does not suffice for the resolution. In this regard, a light-blocking layer comprised of laminated films of low and high insulation materials respectively does not meet the requirement without having a high intraplane insulation.
One of the attempts to solve the problem is a light-blocking layer 14 disclosed in the Japanese Laid-Open Patent Application No. 56419/1989, in which a metal material is scattered like islands over the film surface by using a mask in the formation process, however, in order to satisfy the resolution requirement, the size of the grain (island) of such granular metal film must be smaller than the pixel, this restricts the manufacture of such films, another attempt is a cermet film composed of metal grains dispersed in a dielectric film such as disclosed in the Japanese Laid-Open Patent Application No. 17867/1979, however, in order to have a high impedance(insulation) film, the metal grains must be so dispersed that they are well apart each other, thus it is difficult to manufacture such films without increasing its thickness substantially.